An ultrasonic measurement device includes a wave-transmitter for transmitting an ultrasonic wave and a wave-receiver for receiving an ultrasonic wave, and measures the distance between the wave-transmitter and the wave-receiver based on the amount of time required until the wave-receiver receives the ultrasonic wave after the transmission of the ultrasonic wave by the wave-transmitter. Alternatively, an ultrasonic measurement device measures the distance between an object and the ultrasonic measurement device based on the amount of time required until an ultrasonic wave transmitted from the wave-transmitter reaches the object and the ultrasonic wave reflected by the object is received by the wave-receiver after the transmission of the ultrasonic wave by the wave-transmitter.
In an environment where there are a plurality of such ultrasonic measurement devices, if ultrasonic waves are simultaneously transmitted from different ultrasonic measurement devices, the ultrasonic waves may interfere with one another, thus resulting in erroneous measurement. In view of this, a possible approach is to prevent interference between ultrasonic waves by shifting the timing of ultrasonic wave transmission of each ultrasonic measurement device from those of others (time division transmission).
However, where ultrasonic measurement devices are operating independently of one another, it is not possible to control the timings of ultrasonic signal transmission of different ultrasonic measurement devices so that no interference occurs. In view of this, methods have been devised in which ultrasonic waves are distinguished from one another by encoding, with different codes, ultrasonic waves used by ultrasonic measurement devices.
In view of demodulating and taking out the code of an intended signal from among signals interfering with one another, it is preferred that the other signals are not at all similar to the intended signal. A random signal having such a characteristic that is artificially produced based on a certain rule is called a pseudo-random signal. Binary signals (“1”s and “0”s) are often used for the ease of handling, and pseudo-random signals such as an M-sequence, a Barker sequence and a Golay sequence are known in the art.
An M-sequence, among others, is a code that is used in communications systems using a spread spectrum technique, i.e., a code that serves as a distinguishable, noise-like carrier for information being transferred. This is very effective in taking out an intended signal because between different M-sequences, other signals appear only as noise with respect to the intended signal. Moreover, where signals are both intended signals, if the time of one signal is shifted at all from that of the other signal, they appear only as noise, and it is believed that it is thus possible to know at which point in time an intended signal is present along a sequence of time of interfering receive signals.
Therefore, it is believed that also with ultrasonic measurement devices, by transmitting/receiving ultrasonic waves using an M-sequence pseudo-random signal, each ultrasonic measurement device can perform accurate measurement without being influenced by ultrasonic waves transmitted from other ultrasonic measurement devices, even in a case where ultrasonic measurement devices are operating independently of one another.
Patent Document 1 discloses an ultrasonic measurement device using such an M-sequence pseudo-random signal. Specifically, an M-sequence pseudo-random signal is transmitted on an ultrasonic wave, and a receive signal is obtained by receiving the ultrasonic wave having been reflected by the measurement object. By obtaining the correlation between the pseudo-random signal shifted from the transmission time by a predetermined time interval and the receive signal, the point in time at which the correlation value peaks is obtained as the arrival time of the ultrasonic wave. As the amount of time from the transmission time to the arrival time is the propagation time of the ultrasonic wave, it is possible to obtain the distance to the measurement object based on the propagation speed of the ultrasonic wave.
As described above, between M-sequences, other signals appear only as noise with respect to the intended signal, and therefore there is a very small correlation with a receive signal of an ultrasonic wave transmitted from another ultrasonic measurement device. Thus, no peak is detected in the correlation value, and it is possible to distinguish the pseudo-random signal obtained from the other ultrasonic measurement device.
However, even if encoding is performed using M-sequence discretization, the interference due to cross-correlation increases if the amount of overlap with the ultrasonic wave transmitted from another ultrasonic measurement device increases. If the interference increases, the peak value of the correlation value becomes gentle, and it is no longer possible to determine the accurate ultrasonic wave reception time. As a result, it may be no longer possible to perform accurate distance measurement. If the amount of overlap with the ultrasonic wave transmitted from another ultrasonic measurement device becomes excessively large, it may be no longer possible to obtain the peak value of the correlation value, failing to perform the measurement at all. Moreover, in a case where the energy of the ultrasonic wave transmitted from another ultrasonic measurement device is larger, the peak of the correlation value also becomes vague, and it may be no longer possible to obtain a clear peak.
In the field of radio waves, there is a known method in which the interference is removed by subtracting signals other than the signal of interest from the receive signal in advance so as to realize desirable communications even in such an environment where many of a plurality of radio wave signals interfere with one another.
Patent Document 2 discloses a mobile telephone base station receiver using such an interference preventing technique. The receiver of Patent Document 2 accommodates N (N is a natural number) users, and is capable of removing interference with signals from all users. Referring now to FIG. 22, an example where the signal of user A is taken out from among three users A, B and C will be described.
As shown in FIG. 22, a receiver 80 includes de-spreading sections 22(1) and 22(2) including complex matched filters, demodulation sections 23(1) and 23(2) for obtaining the power of the signal, amplitude ratio section 25(1) and 25(2) for determining the voltage value (threshold value) of a predetermined ratio from the power, extraction sections 27(1) and 27(2) for extracting the waveform using the threshold value, re-spreading sections 28(1) and 28(2), a delay section 30, and a de-spreading section 31. The demodulation section 23(1), the amplitude ratio section 25(1) and the extraction section 27(1) are referred to as a main wave extraction section, and the demodulation section 23(2), the amplitude ratio section 25(2) and the extraction section 27(2) are referred to as a main wave extraction section.
A signal that has been spread with a different code is transmitted from each user (transmitter). Spreading is to temporally expand the information being transmitted (“1” or “0”) using a pseudo-random code such as an M-sequence code. The spread information is transmitted on a sinusoidal wave called a carrier. Different phases (e.g., 0 degree and 180 degrees) or frequencies of the sinusoidal wave correspond to “1”s and “0”s of the encoded signal.
In the de-spreading sections 22(1) and 22(2), the receive signal is subjected to a de-spreading process with codes of users B and C other than user A, which is to be extracted. The de-spreading process is a process in which information that has been temporally widened by a spreading process is brought back to the original by a correlation process with the corresponding pseudo-random signal.
The de-spreading section 22(1) de-spreads the receive signal with the code of user B, and the de-spreading section 22(2) de-spreads the receive signal with the code of user C. The signal produced by de-spreading is a sinusoidal wave corresponding to information transmitted by user B and user C. The sinusoidal wave has received distortion along the propagation path, and also contains thermal noise. Therefore, one cannot know from which point to which point in time is the sinusoidal wave corresponding to the information that has actually been transmitted. Moreover, the propagation path to the base station receiver 80 from the position of user B is different from that from the position of user C.
Thus, a threshold value process for the produced signal is performed by the main wave extraction section. The threshold value process is to extract, from the de-spread signal, a signal greater than or equal to a predetermined ratio with respect to the maximum power of the signal.
Specifically, the demodulation section 23(1) obtains the power (amplitude) information of the de-spread signal obtained from the de-spreading section 22(1). The amplitude ratio section 25(1) obtains the maximum power from the power information, and obtains a voltage value (threshold value) by multiplying the maximum power by a predetermined ratio. The extraction section 27(1) extracts, from the de-spread signal, a signal greater than or equal to the threshold value by the de-spreading section 22(1). The extracted signal is subjected to a spreading process by the re-spreading section 28(1). The thermal noise component is removed by the threshold value process, but the distortion along the propagation path is retained. Therefore, the re-spread signal is a signal obtained by taking into account the profile of the propagation path along which the transmit signal of user B has propagated. Similarly, the transmit signal transmitted by user C is obtained.
An adder 29 subtracts, from the receive signal, the transmit signal of user B and the transmit signal of user C, which are obtained by re-spreading. At this time, the delay section 30 delays the receive signal until signals are output from the re-spreading sections 28(1) and 28(2).
By de-spreading the signal obtained as described above by the de-spreading section 31 using the code of user A, it is possible to suppress interference due to cross-correlation with transmit signals from users B and C, and to obtain a de-spread signal of a high quality.
Patent Document 3 also discloses a mobile telephone base station receiver using such an interference preventing technique as described above. According to Patent Document 3, the base station receiver, which accommodates N users, processes the receive signal with N matched filters, subjects the output to a threshold value process to thereby extract an output greater than or equal to a predetermined level, re-spreads the extracted output with a corresponding spreading code, delays the receive signal so that the receive signal and the re-spread signal are synchronized with each other, and subtracts the re-spread signal from the delayed receive signal to thereby extract the signal from the intended user.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2004-108826    [Patent Document 2] Japanese Patent No. 3476987    [Patent Document 3] Japanese Laid-Open Patent Publication No. 9-200179